This invention relates to a commercial process for the production of alumina and other compounds having a high alumina content from aluminum bearing waste products.
"Alumina," as used herein, refers to a broad range of hydrous and anhydrous aluminum oxides.
It is well known that the processing of aluminum (such as the reduction of alumina to aluminum metal or the melting and/or alloying of aluminum) generates a tremendous amount of waste products. Almost all of these wastes contain varying amounts of aluminum as aluminum compounds, alumina and aluminum alloys. The percentage of impurities in any particular waste product depends on the purity of the starting material. For example, the waste products generated from the reduction of alumina into what is known as pure, prime or primary aluminum metal are relatively free of other metals or oxides of those metals. Conversely, waste products generated from any alloyed aluminum source contain higher and varying quantities of other metals and oxides of those metals. For example, magnesium oxide, calcium oxide and silica are the most common major impurities.
In the past commercial alumina products have been produced primarily from bauxite, mined and converted first to sodium aluminate by the well known Bayer process and then to alumina trihydrate. Because of the purity of the starting materials, this is the most direct process for the production of alumina. Until very recently, other processes which begin with aluminum containing waste products were not economically feasible.
Today, rising energy costs and dwindling natural resources have created a demand for utilization of aluminum containing waste products of the aluminum processing industry. This demand is additionally enhanced by the desire to reduce waste stockpiling and disposal problems.
A previous attempt to recover useful aluminum products from the treatment of aluminum furnace skim, a waste generated in primary aluminum processing, involved crushing the skim and subjecting it to a high temperature steam hydration in a pressurized system. The resulting particles were aluminum metal and other aluminum compounds which were free of nitrides and carbides. See Williams, U.S. Pat. No. 3,660,076.
It is the primary object of this invention to recover substantially all of the aluminum as alumina or high temperature alumina compounds from a chemically and physically heterogeneous aluminum bearing waste product, particularly from secondary aluminum recovery operations.
It is another object of this invention to produce from aluminum bearing wastes commercial alumina for a wide variety of applications other than aluminum metal production.
It is another object of this invention to produce from aluminum bearing wastes a variety of compounds having a high alumina content. For example, such compounds may include: magnesium aluminum oxide; various forms of magnesium aluminate; and magnesium aluminum silicate.
It is another object of this invention to produce alumina and alumina compounds by a low temperature, non-pressurized process.
It is a final object of this invention to convert the aluminum from bag house dust and related waste generated in aluminum dross processing into a variety of commercial products.
The accomplishment of each of these objectives is detailed herein.